Microwave surgery is a technique to selectively coagulate affected part of the body, arrest hemorrhages and perform partial excision utilizing dielectric heat generated in a tissue of the body by a localized microwave electromagnetic field created between electrodes which are inserted in the affected part of the body and between which a microwave of a predetermined frequency is emitted. As to microwave surgery of a deep site of the body, percutaneous microwave coagulation therapy (PMCT) and laparoscopic microwave coagulation therapy (LMCT) are widely applied. A recent development of an open-type MRI apparatus has enabled microwave surgery to be conducted under real-time observation of the affected site utilizing such a MRI apparatus. Microwave electrode devices for this purpose are made of a material substantially inert to the magnetic field created by the MRI apparatus, i.e., a nonmagnetic material so that they do not hinder monitoring of the affected site. Thus, it has been difficult to detect inserted microwave electrode devices on a MRI monitor screen.
The difficulty in detecting a microwave electrode device on the MRI monitor screen, however, causes inconvenience for the surgeon that he or she can not identify, on the MRI monitor screen, the position of the microwave electrode inserted in the body relative to the affected site. To address this problem, a method has been conceived in which a marker of magnetic material is attached near the distal end of the electrode device to induce a magnetic susceptibility artifact (hereinafter referred to simply as an “artifact”), which is an area observed black around the marker on the MRI monitor screen, thereby making it easier to roughly identify, on the MRI monitor screen, the position of the coagulating or cauterizing electrode device in the body. For this, it is proposed to place solder, metal plating such as titanium coating, and a mounted ring member as markers (Japanese Patent Application Publication No. H11-267133).
However, with solder or metal plating, it is difficult to control the amount of the marker to be placed on microwave electrode devices, and, therefore, fluctuation in marker amount among microwave electrode devices is hard to eliminate. Fluctuation among electrode devices in the amount of the placed solder or titanium coat causes fluctuation in the size of the artifact. As a general rule, electrode devices for microwave surgery are disposed of after a single use of them for safety requirements such as prevention of transmission, surgeons are forced to use new electrodes in each operation. A MRI monitoring screen displays only a specific cross section of the body. Consequently, the size of the artifact is greatest when a marker is positioned on this specific cross section, and the artifact reduces its size or eliminates when the marker deviates from the specific cross section as a function of the distance from it. If there is fluctuation in the size of generated artifact among electrode devices, a surgeon, who manipulates an electrode device relying on the position and the size of the artifact, must identify in every operation the maximal diameter of the artifact induced by the electrode device used while using the very electrode device and then reflect this to his manipulation. This causes a great deal of inconvenience and impedes achieving the uniformity of manipulation.
In addition, in order to keep coagulated tissues from adhering to the microwave electrode device being used, a fluorocarbon resin is coated on the electrode device. As the baking temperature for this (about 390° C.) is what solder (melting at about 200° C.) cannot stand, use of solder as a marker would cause a trouble in application of fluorocarbon resin coating. Furthermore, when water is lost from tissues being cauterized in an operation, the electrodes could reach high temperatures, and this, due to the melting of the solder, could lead to risks of leaking and falling of the solder on the lesion.
In the case where a ring member is to be mounted as a marker, there is a problem of how to securely fix the member to prevent it from loosening or falling. That is, in microwave surgery now, needle-like devices are used because of year-to-year request for thinner, needle-like electrode devices. Thus, where an electrode with small diameter is inserted in a ring-shaped marker member, it would become necessary to securely fix the marker member onto the electrode. A screw or other components cannot be applied, however, due to the small diameter of the electrode. As for welding, arc welding cannot be applied, for it would not only cause an uncontrollable extra artifact due to added welding materials but also lead to reduction in the strength of the electrode due to residual stress. Although application of spot welding could evade the problems of an extra artifact and reduction in the strength, it is difficult to securely fix the ring-shaped member by spot welding alone. Otherwise, spot welding applied in excess also could lead to reduction in the strength of the electrode due to remaining stress. Moreover, for easiness of an operation, a marker is to be mounted preferably onto the central conductor body at the tip of the electrode device. However, because of the narrow diameter of the central conductor body, it is much more difficult to securely fix a marker ring onto the central conductor body. And further, when the tip of the electrode device is burnt and bonded to a cauterized tissue, it is necessary to manipulate the electrode device to detach its tip from the biotissue, in the process of which stress is concentrated on the tip of the electrode device. In case the marker ring is not securely fixed onto the central conductor body, it could lead to an event that the marker detaches due to the stress and falls together with the central electrode covering the marker.
As electrode device, as a general rule, is to be disposed of after each operation, means for securely fixing the marker must be simple and applicable at low costs so as not to be a factor that could cause a price rise of the device.